Russia is the bigger problem currently, most likely. China isn't trying to conquer its neighbors, just one of them, and also the South China Sea. The Russian people, however, really do want to re-establish a Russian empire of some sort. They don't really have the economy or means to do so, but they do have a bunch of nukes (which may or may not be fully operational) to cause a lot of damage with, and they have a geriatric dictator with possible mental problems who they wholeheartedly support.
I don't see why it's even a dichotomy as pushed by GP. Russia is now the junior partner in the Russia-China relationship against the US/EU international order (as opposed to the Cold War). Putin is actively testing the strength of that international order, Xi is mostly passively watching and biding time. The outcome of Russia-Ukraine will most certainly determine how aggressively China acts going forward.
I don't see how: China only started in 1949. The USSR is older than that, while the USSR's zombie reincarnation is only about 30 years old. If you're talking about historical China, then you might as well talk about Genghis Khan or the founding of the city of St. Petersburg or whatever; it's kinda pointless because modern nation-states don't resemble those ancient societies at all.
I’m not sure China was ever particularly interested in conquering foreign countries now or historically (not saying Taiwan is not a real country but it’s not really foreign and certainly an exception in most ways).
On the other hand violent imperialism has been a core aspect of Russian culture for centuries.
Historically, countries gave up on their imperialism after major defeats. That's one reason Russia can't come out of this war with a win, it only invites repetition.